This invention relates to a semiconductor laser device having an active cavity possessing a gain and a passive optical waveguide integrated on the same substrate, and is intended to provide novel effects including stable single longitudinal mode oscillation, narrow spectral linewidth, a small amount of wavelength chirping due to current modulation, and no generation of intensity of noise against reflected light.
The recent progress of the semiconductor laser is striking, and it is widely used as the light sources for optical communication, optical information processing, optical fiber sensing, and so forth. At the present, however, the semiconductor laser involves the following four problems.
Firstly, the semiconductor laser does not oscillate in a single longitudinal mode. In an ordinary Fabry-Perot type semiconductor laser using cleaved facets as cavity mirrors, several longitudinal modes are likely to oscillate at the mode spacing determined by the cavity length (for example, 300 .mu.m) due to the broad semiconductor gain width. That is, there is a disadvantage of occurrence of multimode oscillation.
Secondly, the spectral linewidth of the semiconductor laser is fairly broad. The spectral linewidth of a laser is usually defined by Shawlow-Townes' equation, but in the case of a semiconductor laser, in particular, it is shown that the linewidth is actually extended by (1+.alpha..sup.2) times the value described by this equation by, for example, C. H. Henry in "Theory of the linewidth of semiconductor lasers", IEEE J. Quantum Electronics, Vol. QE-18, No. 2, pp. 259-264 (1982). That is, in other words, there is a disadvantage of an extremely short coherent length.
Thirdly, the dynamic spectral linewidth is broad. For intensity modulation of a semiconductor laser, the injection current is modulated, which, however, results in fluctuations of carrier density and large variations of oscillation wavelength (oscillation frequency). This phenomenon is called wavelength chirping. For instance, when a semiconductor is used as a long-haul optical fiber communication source, the transmittable distance is significantly shortened if the amount of chirping is large, which is another disadvantage of the semicondutor laser.
Fourthly, an extremely large amount of noise is generated when a reflected light returns to the semiconductor laser from outside components. That is, when a semiconductor laser is coupled with an optical fiber or it is used as a source of optical information processing such as an optical disc, substantial reflected light returns to the semiconductor laser, noise increases, and the S/N ratio of the unit worsens, which is a great shortcoming for practical use.
Accordingly, solutions of these four problems are keenly demanded by users, and several methods have been proposed so far.
For example, a distributed feedback (DFB) laser is known, as presented by S. Akiba et al., "Low-threshold-current distributed-feedback InGaAsP/InP CW lasers," Electron. Lett., vol. 18, pp. 77-78 (1982). In the DFB laser, a sufficient characteristic is obtained as to the single longitudinal mode oscillation, but satisfactory results are not necessarily obtained with respect to spectral linewidth, oscillation frequency chirping and intensity of noise due to reflected light. That is, all four problems above are not solved completely. A cleaved-coupled-cavity (C.sup.3) laser was proposed by W. T. Tsang et al., "high-speed direct single-frequency modulation with large tuning rate and frequency excursion in cleaved-coupled-cavity semiconductor lasers," Appl. Phys. Lett., vol. 42, pp. 650-652 (1983). This solution fabricates a coupled cavity laser by cleavage using two active cavities, but it involves problems in mechanical stability and reproducibility, and it is also reported that the characteristics are impaired when the reflected light is fed back. Again, the four problems are not solved completely. Another method was proposed by H. K. Choi and S. Want, "GaAs/GaAlAs active-passive-interference laser," Electronics Letters, vol. 19, pp. 302-323 (1983). This semiconductor laser is formed by using an extremely short optical waveguide as a coupled cavity. The purpose of this laser lies solely in the unification of the longitudinal mode by mode selectivity of the cavity, and this method, which will be described in detail later, does not narrow the spectral linewidth or suppress wavelength chirping against current modulation. Further, this solution does not refer to noise at all, and it is far from solving all of the above four problems.
In this background, the present inventors have invented, for the first time, a semicondudtor laser of a novel structure that can solve all of such four problems, that is;
(1) stable single longitudinal mode oscillation
(2) narrow spectral linewidth
(3) suppression of wavelength chirping due to current modulation
(4) low noise
all at once. This invention is based on U.S. patent application Ser. No. 671,469 entitled "Oscillation Frequency Stabilized Semiconductor Laser" (Filed Nov. 14, 1984, now U.S. Pat. No. 4,677,630 U.S. application Ser. No. 671,469 does not teach a monolithic structure and its detail as a practical device. In this invention, a semiconductor laser of monolithic structure was actually fabricated, and its characteristics were evaluated experimentally, the results of which are displayed here to show the effectiveness of the present invention. At the same time, the philosophy of the present inventors in solving the above four problems simultaneously, supporting evidence and possible examples of application are also explained.